The present invention relates to an interface device between a broad-band ultrahigh frequency (hyperfrequency or microwave) optoelectronic sensor and a load.
It applies in particular to the production of ultrahigh frequency monolithic integrated circuits, which are designed in particular for transmission of very broad-band ultrahigh frequency signals on optical fibres.
In general, optoelectronic sensors, such as photodiodes, act as a source of current which is controlled by modulation of the incident light, in which a parasitic capacitor C is mounted in parallel on a resistor with a very high value.
From the point of view of adaptation of impedance, the ideal load (which is also known as the reading resistance) to be submitted to this photodiode would be a resistance R with a high value. Since the cutoff frequency is proportional to the time constant RC, depending on the photodiode used, a load of this type, associated with the parasitic capacitor C, could give rise to a cutoff frequency which is much lower than the maximum frequency of use of the ultrahigh frequency optical connections.
A means for reducing the value of the time constant RC consists of decreasing the resistance R of the load. For example, it is known to decrease this resistance to a value lower than 50 ohms, by connecting the photodiode to an amplifier with a low input impedance. However, a solution of this type has the disadvantage that it decreases greatly the gain of an optical connection chain, and in some cases increases the noise factor, since the said noise factor is a decreasing function of the resistance R, the increase in the noise factor being all the most substantial, the lower the power which is incident on the photodiode.
In addition, in EP-A-801466, the applicant has proposed an assembly known as the bootstrap, in which the negative effects of the parasitic capacitor of the photodiode are compensated for by cancelling out the difference in potential at the terminals of the said parasitic capacitor. In practice, this bootstrap comprises a field-effect transistor mounted in a common drain, the gate of which is connected to one terminal of the sensor, and the source of which is connected to the other terminal of the sensor.
In addition, the bootstrap is completed by an impedance adaptation stage which comprises another transistor mounted in a common drain. The gate of the transistor of the adaptation stage is connected to the source of the transistor of the bootstrap, and the source of the transistor of the adaptation stage is applied to the standardised impedance load of 50 ohms.
An impedance adaptation stage of this type is not entirely satisfactory, in that the useful signal is recuperated in this case on the source resistance of the transistor of the bootstrap. In addition, a connection capacitor is interposed between the source of the transistor of the bootstrap and the gate of the impedance adaptation transistor, and an impact inductor is placed directly in parallel on the load.
As a result, amongst other disadvantages, there is a limitation of recuperation of the useful signal, towards the low frequencies.